Bias circuit

Abstract

Proposed is a bias circuit for a transistor in a C class amplifier. The bias circuit comprises: a class AB amplifier bias voltage generating means adapted to generate a bias voltage at an output terminal; and a transistor connected between the output terminal and a first reference voltage, the control terminal of the transistor being connected to a second reference voltage via a switch. Closure of the switch connects the second reference voltage to the control terminal of the transistor to cause a shift in the bias voltage generated by the class AB amplifier bias voltage generating means to achieve a predetermined class C bias voltage at the output terminal.

Description

FIELD OF THE INVENTION[0001]This invention relates to a bias circuit for a Radio Frequency (RF) power transistor.CROSS-REFERENCE TO RELATED APPLICATIONS[0002]This application claims the priority under 35 U.S.C. §119 of European patent application no. 13290021.8, filed on Jan. 28, 2013, the contents of which are incorporated by reference herein.BACKGROUND OF THE INVENTION[0003]In the field of RF power amplification, with the increasing complexity of signals to be amplified (such as GSM, WCDMA, LTE, etc. . . . ) and the demand for efficiency, the preferred mode of operation of a RF transistor is a common source (emitter) class AB linear amplifier.[0004]In order to maintain linearity, it is known to use a bias circuit to provide thermal compensation. This has been implemented in MMIC's using the well-known current mirror arrangement of transistors. When integrated on the same die as a RF power transistor, this provides thermal tracking and process compensation.[0005]With the continued ...

AI Technical Summary

Benefits of technology

[0008]There is proposed a bias circuit that employs a transistor connected between the output of the bias circuit and a reference voltage (such as ground, for example). The transistor may be adapted to act as a non-linear load and may allow quick discharge of load capacitance in a switching mode. Thus, embodiments may provide fast thermal tracking and process compensation together with low video impedances. Embodiments may also be compact in size, exhibit lower power consumption, and provide a reduced switching time (such as below 100 ns for example). Embodiments may therefore be employed in MMIC's or discrete transistors, and will be suitable for Doherty amplifiers and Time Division (TD)) amplifiers.

[0015]Embodiments may exhibit low impedance to prevent parasitic modulation appearing at the output (e.g. at the input of a transistor to be biased), thus restricting the generation of undesirable memory effects and non-linearity. Embodiments may compensate for part-to-part variation in the threshold voltage of an amplifier's transistor, thus reducing a need for adjustment of the bias circuit.

Problems solved by technology

A conventional current mirror (like that shown in FIG. 1 for example) is therefore not suitable for Class C.

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